Logging

Implement simple logging based on Boost.Log. Log messages can have different severity, and filtering on it is supported. The run-time overhead for messages that do not pass the filtering is minimal. The filtering is controlled with verbosity level set via a command line option. By default, messages with severity info and above are displayed.

All of existing logging output is preserved, but much of it has been moved to the debug severity level. It will not be displayed with the default verbosity level.

@npostiau, @hanwen, please note this update and use the logging in future. In particular, consider adding debug- and trace-level messages generously. They won't be displayed with the default verbosity and so won't clutter the output, but will be useful for debugging. Basically, there should be no reason to comment out debug output.